Indian ink consists of purified soot mixed with gum, as binding medium, and a little camphor and musk. It is generally believed that Indian ink is made in China by a process which is still a secret; this opinion is supported by the fact that the ink made in Europe is almost always inferior to the Chinese. It is quite possible that a substance which produces a particularly fine black is used in China to prepare the soot used for the ink, and which we do not yet know or do not use on account of its high price. But it appears that the difficulty of producing an ink equal in quality to the Chinese lies less in the quality of the soot —for by the methods of purification already given we can make almost chemically pure carbon—than in the extremely careful mechanical treatment of the soot with the other constituents of the ink.

The genuine Chinese ink has a peculiar smell, which, when not hidden by musk, distinctly recalls the smell of burning camphor; the camphor tree is a native of China, so that it is not impossible that the soot of the wood of this tree is used for the ink. The soot from camphor alone would be too expensive, so that it is probable that camphor wood is used from which camphor has been prepared, or that camphor soot is mixed with the soot of a cheaper substance. In fact several varieties of Chinese ink show distinctly the smell peculiar to soot produced by fat. The binding material is animal glue, which is most thoroughly ground with the carbon; the secret of making good Indian ink lies in the careful grinding with the binding medium.

According to Piou, who lived for a long time in China and learned the process there used, the black employed for the ink is made from the resins of the pine and other trees. The soot is sieved through silk, boiled with glue and water, and kneaded by hand with a little oil until the mass has become completely homogeneous. It is then left for some days, heated, and pressed into the moulds. This description is not very satisfactory; the products obtained are far inferior to those made by the ordinary European process.

The viscous mass obtained by long treatment of the black with the binding medium is slowly dried, and again ground when it has become thick. When a completely homogeneous paste has been obtained it is made into sticks which are pressed into moulds, and then very slowly dried so that they do not crack. Small cracks always occur; they are filled in by a brush dipped in the thick paste, finally the sticks are entirely or partially covered with gold leaf.

In order to impart to the ink the odour characteristic of the Chinese ink, it is sufficient to use good lamp black, made odourless by extraction with caustic soda, and to add a little solution of camphor in turpentine during the grinding. If it is desired to impart the musk smell which several Chinese varieties possess, a small quantity of a spirit extract of genuine musk may be added.

The size solution is made by long boiling isinglass with water; it must be so concentrated that it forms a strong jelly on cooling. A small quantity of acetic acid is added, so that the grinding may not be hindered by the viscous nature of the size, which the acid prevents from gelatinising. When the mixture has been made by long grinding, the mortar is warmed to 40° or 50° C., whereupon the acetic acid is soon volatilised, and the mass rapidly becomes very thick.

Neutral Tint Black.—A mixture of Indian ink, Chinese blue, and a very small quantity of madder lake forms the colour known as ”neutral tint,” the shade of which is a peculiar greyish violet. By alterations in the proportions of the constituents different shades of ”neutral tint” are produced.

Appendix.—Black Mineral Pigments.

Chrome Copper Black.—When copper chromate is strongly ignited in air and then treated with boiling nitric acid a glittering black compound is obtained, which shows up well when printed on fabrics with albumin. This and all similar pigments are distinguished by great durability.

Chrome Black.—When a mixture of chromium oxide with varying quantities of ferric oxide is strongly heated, pigments are obtained of all shades from dirty yellow and green to the deepest black. This pigment is largely used in porcelain painting to produce a black, which also can be obtained by other but much more costly methods. The best mixture to produce a deep black contains one part of chromium oxide to four parts of ferric oxide.